Chapter 4 : Weight and Balance

In Chapter 2, we had a brief lesson on how weight affects the aircraft in flight. Because gravity affects the aircraft weight, it is of utmost importance to keep the weight to a minimum. Also, it is important to determine the distribution of weight. Although the aircraft has normal or below the recommended weight, we need to achieve stability for the aircraft to fly properly. If not, the aircraft is hard to control, which may result in unnecessary crash. The distribution of weight is essential to balance the aircraft. Since majority of the aerodynamic forces acts on the wing, it plays a major role in controlling the aircraft.

Looking at the cross-section of the wing, the aerodynamic center is the pivot point of the lateral axis (see Fig 16a). The center of gravity is where all the aircraft weight is concentrated which must be near the aerodynamic center to achieve stability. Since the airplane has two sets of wings (main and tail wing) there are two aerodynamic centers to be considered. The combination of two aerodynamic centers is the neutral point ( see Fig. 16b ). If the center of gravity is way behind the neutral point, the aircraft is called tail-heavy which might result in stalling. If its located too far forward, nose-heavy will result and it will be too hard for the aircraft to gain altitude (see Fig. 16b).

Figure 16a : Aerodynamic center of an airfoil

Figure 16b : Neutral Point of Wing and Horizontal Stabilizer

In a full-size aircraft, the center of gravity determination has to be calculated. The arm or the distance of the load (or cargo) multiplied by the weight is called the moment (remember physics?). The fulcrum or the pivot point is the suggested center of gravity location of the aircraft. The moments of the two sides should be equal so as to balance the aircraft (see Fig. 17a). Since our model aircraft is miniaturized, all we need to do is to balance the aircraft with our fingers on the recommended location of the center of gravity. Based on the example illustration of Fig. 17b, the C.G. (or center of gravity) location is 2 inches from the leading edge.

Figure 17a : Illustration of weight and moments

In model aircraft, the distribution of weight depends on the location of the radio equipment installed. Because they are the last thing to be installed and does not affect much the aerodynamic characteristics of the aircraft The servos, batteries and receivers should be located in such a way that the aircraft will balance on the center of gravity. The engine location was already designed on most plans or kits available so there is not much we can do about it. The best thing to do is before permanently installing the equipment is to arrange it temporarily with a tape then balance the aircraft. The model should already be completely built (engine & fuel tank installed, the airframe is already wrapped with monokote, etc). When the desired location is known, then you can permanently install the radio equipment.

Figure 17b : Balancing model airplane by hands

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